Audio Demodulation And Power Amplifier - Hitachi CP2155TA Service Manual

AUDIO DEMODULATION AND POWER AUDIO DEMODULATION AND POWER
AMPLIFIER AMPLIFIER
FM Demodulation
Video and inter-carrier FM is output from pin 6 of IC504.
FL601 Traps the FM carrier from the video signal.
Composite video is then buffered by TR503 and passed to
the rest of the chassis for processing. Prior to the FM trap
(FL601), the FM carrier(s) are passed to the audio PWB for
decoding via pin 2 of PL603. FM Demodulation is then
carried out on the daughter PWB inside IC6201.
ZWEITON Decoding.
The Zweiton audio is decoded on a separate PWB. This
PWB also carries out audio switching and tone, bass,
balance, and volume control functions via I²C.
Inter-carrier FM is passed to the Dual FM Demodulator
IC6201 ( U2860B ) via pin 2 of SK6201, ceramic filters
FL6201-FL6204 selecting the two FM audio carriers (at
5.500 & 5.741 MHz respectively) required. This IC
demodulates one, or for a dual language or stereo signal –
both FM carriers. These are fed to the Zweiton (A2) decoder
IC6202 pins 7 and 8, where the pilot 54KHz signal on the
second carrier is detected and demodulated across the tank
circuit comprising L6203 and C6215 on pins 4 and 5. The
audio mode is then determined so that the appropriate audio
signals are produced on pins 13 and 14.
The audio signals are then routed to pins 3 and 5 of IC6203,
and SCART 1, pins 1 and 3 of SK501 via buffers TR6203 &
TR6204.
Volume/Tone control
Volume, tone control, & SCART audio switching are all
carried out on the audio PWB by IC6203.
The broadcast audio signals are routed to pins 2 and 5 of
IC6203; SCART 1 audio inputs from SK501 ( pins 2 & 6 ) are
routed to pins 32 and 1; and AV2 inputs from the White and
Red Phono sockets of SK6401, are routed to pins 28 & 30.
IC6203 performs all the audio switching, volume, balance,
bass, and treble functions via I²C. The processed audio
signals are then routed to the loudspeaker power amplifier
via pins 11 and 12 of SK6201.
Headphone facilities, are also controlled by IC6203; this
adjusts volume and balance only for the headphone output.
The audio signals are boosted by the low power headphone
amplifier IC6204, and sent to the headphone socket
SK6601. Additional components on the AV2 & Headphone
socket PWB de-couple the output signals and provide EMI
filtering
Audio Power Amplifier
The audio power amplifier is located on the main PWB and
is designed to output 6.0W at 10% distortion into 8 ohm
speakers. The processed audio is routed to pins 2 and 4 (left
and right respectively) of the audio power amplifier IC603
from the audio PWB via pins 11 and 12 of PL603. The power
amplifier outputs at pin 12 (left) and pin 7 (right) feed the left
and right loudspeaker plugs PL601 and PL602 respectively,
through CE614 and CE615. High frequency 'snubber'
networks R614, C617 and R615, C618 are fitted.
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